Soft switching in flyback converters using a fixed dead time

ABSTRACT

Soft switching in flyback converters using a fixed dead time is presented. Embodiments disclosed herein relate to switching circuits and more particularly to soft switching in single stage isolated flyback converters. Embodiments herein disclose systems for soft switching in single stage isolated flyback converters operating in boundary control mode, by incorporating a fixed dead time.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and derives the benefit of Indian Provisional Application 201641036752, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

Embodiments disclosed herein relate to switching circuits and more particularly to soft switching in flyback converters.

BACKGROUND

Flyback converter topology is the basic power conversion scheme for AC-DC isolated converters for high voltage and low output current requirements. Mostly these are limited to around 200 watts output power and multiple output requirements. However, in a normal flyback converter, the stresses on the components are higher due to parasitics associated with magnetics. A hard switched flyback converter invariably demands an RCD snubber to protect the main switching device, be it a metal-oxide-semiconductor field-effect transistor (MOSFET) or a bipolar transistor. This is mainly due to leakage inductance associated with the power transformer. MOSFET is widely used for high frequency applications in the range of 100 KHZ. In low input voltage applications, input currents drawn are fairly large. To reduce the conduction losses, designers choose MOSFETs, which have a very low R_(DS(on)). Most of the MOSFETs having a very low R_(DS(on)) values, also have larger capacitances associated with them. This restricts the usage at high frequencies because of the higher switching losses. In order to use the low R_(DS(on)) devices at high frequency, it is mandatory to adopt soft switching techniques. Soft switching mitigates the switching losses and hence allows high frequency operation.

Most of the existing soft switching techniques use an additional auxiliary switch and its drive control to achieve soft switching. Additional components and drive control are of a higher complexity and cost.

OBJECTS

The principal object of embodiments herein is to disclose systems for soft switching in single stage isolated flyback converters by operating the converter in boundary control mode and then by only incorporating a fixed dead time.

BRIEF DESCRIPTION OF FIGURES

Embodiments herein are illustrated in the accompanying drawings, through out which like reference letters indicate corresponding parts in the various figures. The embodiments herein will be better understood from the following description with reference to the drawings, in which:

FIG. 1 depicts a flyback converter with an isolated low voltage output, according to embodiments as disclosed herein;

FIG. 2 depicts the timing sequence of the scheme, according to embodiments as disclosed herein; and

FIGS. 3a, 3b, 3c, 3d and 3e depict example observations and values, according to embodiments as disclosed herein.

DETAILED DESCRIPTION

The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

The embodiments herein disclose systems for soft switching in single stage isolated flyback converters operating in boundary control mode, by incorporating a fixed dead time. Referring now to the drawings, and more particularly to FIGS. 1 through 3 e, where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments.

Embodiments herein disclose soft switching in flyback converters using parasitic elements. Embodiments herein disclose a methodology to achieve complete soft switching in an isolated flyback converter for a variety of applications and line/load conditions. Embodiments herein incorporate a fixed dead time in a boundary conduction mode control scheme, which accomplishes the soft switching of the power devices, wherein an auxiliary power switch and its intended control are eliminated.

Embodiments herein provide complete soft switching in isolated single stage PFC (Power Factor Correction) flyback converters. Embodiments herein incorporate a fixed dead time in a boundary conduction mode control scheme, which enables total soft switching.

FIG. 1 depicts a circuit 100 for a flyback converter with an isolated low voltage output. The circuit 100 has a single output V_(o) isolated from the input. The circuit 100, as depicted, comprises of a leakage inductance L_(L) 102 and an effective capacitance C_(eff). C_(eff) comprises of capacitance of a SW 103, inter-winding capacitance of a transformer (T) 104, the junction capacitance of the diode D_(s) 105, and a capacitor C_(r) 101. C_(r) 101 is present across the switch SW 103. SW 103 is the main power switch and the ON and OFF time duration of the SW 103 regulates the output voltage across the capacitor C_(o) 106. SW 103 is connected in series with the primary windings of T 104. In an embodiment herein, SW 103 can be at least one of a MOSFET switch, a bipolar transistor, and an insulated-gate bipolar transistor (IGBT). Embodiments herein are further explained considering that SW 103 is a MOSFET switch, but it may be obvious to a person of ordinary skill in the art that any other form of suitable switch may be used (such as a bipolar transistor, an IGBT, and so on). L_(p) is the primary inductance of the transformer 104 comprising N_(p) turns. L_(s) is the secondary inductance of the transformer 104 with turns N_(s). D_(s) 105 is the secondary side rectifier. D_(s) 105 is connected to the secondary windings of T 104.

A control IC can achieve boundary current control mode. An internal multiplier in the IC coupled with output voltage feedback ensures the unity power factor and regulates the output voltage. The boundary control mode ensures that the input current follows the input voltage. Boundary conduction mode is detected with a separate sense winding in the transformer 104, which generates a negative going edge (positive to negative transition) at the instance of secondary current falling to zero.

Since the converter is operating in the boundary current control mode, the turn ON current in the SW 103 is zero, which is a ZCS (Zero Current Switching) at turn ON. Also, the D_(s) 105 turns OFF with ZCS and hence there is no reverse recovery issue with the diode.

C_(r) 101 absorbs the energy in the leakage inductance of the transformer 104, when SW 103 is turned OFF. This in turn enables avoidance of a dissipative turn OFF snubber. C_(r) 101 can also aid in achieving ZVS (Zero Voltage Switching), while the power switch is turned OFF. C_(r) 101 resonates with the primary inductance and aids in securing ZVS at turn ON due to incorporation of adequate dead time.

The peak current of the SW 103 at the end of T_(on) for DCM (Discontinuous Conduction Mode) and BCM (Boundary Conduction Mode) (I_(pp)) can be

I _(pp) =T _(on) *V _(in)/(L _(p) +L ₁)   (1)

Consider a time period t₀ to t₁. Prior to t₀, SW 103 is ON for a period of T_(on) and just at t₀, SW 103 is turned off. Stored energy shall not immediately be transferred to output due to resonance between L_(p)+L₁ and C_(eff). The circuit is viewed as a series resonant circuit with initial current in inductance as I_(pp) and initial voltage at C_(eff) as zero.

The excitation voltage is the input voltage V_(in). t₁ is the instance at which drain voltage at SW 103 reaches maximum value and the current in the resonant circuit is zero. Under such conditions, instantaneous current in the circuit i(t) and the voltage at drain of SW 103, v(t) during t₁−t₀ interval can be written as

i(t)=0.5*I _(pp)*(1+cos(p(t−t ₀)/(t ₁ −t ₀))−2 sin(p(t−t ₀)/(t ₁ −t ₀)   (2)

v(t)=0.5 I _(pp)*√((L _(p) +L ₁)/C _(eff))*(1−cos(p(t−t ₀)/(t ₁ −t ₀))   (3)

Under unclamped conditions of a resonant circuit, the voltage across C_(eff) shall reach the maximum value of

I_(pp)*√((L_(p)+L₁)/C_(eff))   (4)

However in the given flyback configuration when this voltage reaches a value,

Vin+(V0/Ns)*Np   (5),

the resonance stops because the output rectifier (D_(s) 105) conducts and the stored energy in the primary of the transformer (T) 104 begins to flow to the load. This is the energy transfer phase to output load. However, at the primary side, the drain voltage continues to rise due to the energy stored in the leakage inductance and I_(pp). Since leakage inductance is orders lower than L_(p), voltage at the drain, after reaching a higher peak value, settles down quickly to the value

V_(in)+(V₀/N_(s))*N_(p)   (6)

This peak voltage should be limited to a safe value well below the absolute maximum voltage of the switch SW 103.

In most of the practical applications, the BCM is implemented with variable switching frequency. The switching frequency varies with line and load. Current in the secondary is sensed and the moment a zero current is detected during the SW OFF period, the primary switch (SW 103) is again turned ON.

If the turns ratio of the transformer (T 104) is so chosen that the reflected voltage at the primary is at least equal to V_(inmax), during the energy transfer phase, then the duty cycle (D) is 0.5 for V_(inmax) input. For any input voltage Vin,

D=V _(inmax)/(V_(in) +V _(inmax))   (7)

Average current in the primary turns of the transformer (T) 104 is

I _(p)avg=I _(pp)*0.5*D   (8)

The average current in the secondary turns of the transformer (T) 104, which is same as load current I_(L) is given by

I _(L) =I _(sp)*0.5*(1−D)   (9),

where I_(sp) is the peak secondary current.

The turns ratio of transformer (T) 104 is selected in such a way that the reflected voltage at the primary is at least equal to highest input voltage, while the secondary diode D_(S) 105 is conducting. This ensures sufficient energy to be stored in the C_(r) 101, which will drive the ZVS condition at turn ON.

A fixed dead time is introduced before turning ON the SW 103. The dead time should be set to be equal to the half of resonant time period of primary inductance L_(p) and C_(r) 101.

The voltage at C_(r) 101 will be at least equal to twice the input voltage, when D_(S) 105 is conducting. Once the stored energy in T 104 is fully delivered to the load, D_(S) 105 turns OFF with zero current. Then, the primary circuit can be considered as a resonant LC circuit with initial current in LP as zero and voltage at C_(r) 101 as twice the input voltage. The energy in C_(r) 101 initially is transferred to the primary inductance till C_(r) 101 voltage reaches the input voltage. Subsequently, the energy in the primary inductance starts discharging C_(r) 101 down to zero value.

This is the favorable time to turn ON SW 103, which realizes the ZVS. Thus if a delay/dead time is added to the instance of detection of zero current and then allow the SW 103 to turn ON, in the boundary control scheme and the delay being equal to half the resonant time of primary inductance and C_(r) 101 then, total soft switching is achieved.

FIG. 2 depicts the timing sequence of the scheme. The timing sequence can be divided in to time intervals and explained as under:

T₀-T₁: Prior to T₀, the power switch SW 103 is off and at T₀, the SW 103 is turned ON. Since the converter 100 is in boundary control mode, the initial turn ON current is zero and hence ZCS at turn ON. The primary current linearly ramps up. This mode ends at T₁ when the switch 103 is turned OFF. This period is same as T_(on). T₁-T₂: Because of C_(r) 101, the voltage across SW 103 is held at zero value at T₁ achieving ZVS at turn OFF. The current in the primary windings of T 104 is diverted to C_(r) 101 and resonance starts between L_(p) and C_(r) 101. This mode ends at T₂ when the drain voltage of SW 103 reaches a value V_(in)+(V₀/N_(s))*N_(p). Neglecting the leakage inductance effect, drain voltage settles to V_(in)+(V₀/N_(s))*N_(p) and the energy stored in the primary windings of T 104 begins to flow to the secondary windings of T 104. T₂-T₃: This is an energy transfer phase and is equal to T_(off). T₃-T₄: This period is equal to half the dead time designed and at T₃, the secondary current has fallen to zero value. The drain voltage of SW 103 is at V_(in)+(V₀/N_(s))*N_(p) and the resonance starts again between L_(p) and C_(r) 101. The initial current in the resonant circuit is zero. Energy stored in C_(r) 101 at V_(in)+(V₀/N_(s))*N_(p) voltage is initially transferred to L_(p) and the voltage at C_(r) 101 reduces sinusoidally. This mode ends at T₄ when the C_(r) 101 voltage reaches V_(in). T₄-T₅: Since the voltage across L_(p) reaches to zero value at T₄, the current in L_(p) tries to remain constant by circulating in the V_(in) and C_(r) circuits. However since the C_(r) 101 is not a voltage source, its voltage starts reducing sinusoidally and reaches a value zero at T₅. The duration of T₅ to T₄ is equal to half the dead time designed and hence SW 103 turns ON again at T₅, achieving ZVS at turn ON. Thus total soft switching is achieved for the SW 103. T₅ and T₀ are the same. The time gap between T₅ to T₃ is the designed dead time equal to half the resonant time of L_(p) and C_(eff).

Single stage isolated converters with active PFC are best suited for low and medium power applications. In such configuration, Boundary Conduction Mode flyback converters are widely used and many control ICs are commercially available. The high voltage input filter capacitor is eliminated and hence there is no inrush current, obviating the need for associated protection circuit. In such a scheme, embodiments herein achieve soft switching to further enhance the efficiency.

A flyback converter with PFC was built adapting the BCM. Subsequently, a fixed dead time was introduced after detecting zero current in the transformer before turning ON the power switch. This achieved the total soft switching for the MOSFET.

The following section details an example computation of component values for a practical 100 W flyback converter to achieve complete soft switching in the BCM.

Input voltage: 140 to 280 VAC or 200 to 400 VDC Output voltage: 135 VDC

Output Current: 0.75 Amps.

MOSFET should have at least 1000V breakdown capacity. Assuming an efficiency of 90%, the average input current is 0.37 Amps at an input of 300 VDC. In the boundary mode control, as stated in equation (7) The duty cycle at 300V DC is equal to 0.57,

Hence, the primary peak current is

I _(pp)=0.37*2/0.57=1.3 Amps.

From the data sheet of the MOSFET, the turn off time t_(off) is about 50 nS. Therefore, if the rise of drain voltage to its final value is delayed by t_(off), zero voltage switching at turn OFF can be achieved. Hence the value of C_(r) 101 can be arrived as

C _(r) =I _(pp) *t _(off)/700V=93 pF, choose 100 pF   (10)

The computation of C_(r) 101 becomes important, because the ultimate delay to be incorporated after detecting boundary condition is dependent on C_(r) 101 and this delay in turn should not generally exceed about 10% to 15% of the total switching time T at the maximum frequency condition. Longer delays cause higher peak currents and hence higher conduction losses.

Turns ratio of the transformer is

N=N _(p) /N _(s)=400/135=˜3.0   (11)

Peak current in secondary is

I _(sp)=1.3*3=3.9 Amps   (12)

Fixing a maximum frequency as 125 KHz at the maximum input voltage and fixing a duty cycle of 0.5 and hence the ON time T_(ON) is 4 μS.

Peak input current at maximum input voltage is 1.12 Amps. Therefore, Primary inductance L_(p) is computed as

L _(p)=400*4*10⁻⁶/1.12=1.43 mH   (13)

The delay time required to be incorporated after detecting boundary mode is

p*√(1.43*10⁻³*100*10⁻¹²)=1.16 μS   (14)

The design values arrived are

L_(P)=1.43 mH C_(r)=100 pF

Delay time=1.16 μS Maximum switching frequency=125 KHz. Switching frequency at V_(inmin) and full load is 55.5 KHz.

With the above values, a flyback converter was built and evaluated. All the observations are documented and presented in the FIGS. 3a, 3b, 3c, 3d and 3e . FIG. 3a depicts the input AC voltage and current waveforms. FIG. 3b depicts the drain and gate waveforms for the BCM flyback converter with dead time. FIG. 3c depicts the zero voltage turn ON and zero voltage turn off. FIG. 3c depicts the input current harmonics for the BCM flyback converter with dead time. FIG. 3d depicts the efficiency at various DC input voltages and loads. FIG. 3e depicts the efficiency and power factor at various AC input voltages and 100 W load.

To benchmark the advantages of the proposed scheme, the flyback converter was redesigned to operate in the continuous conduction mode (CCM) with 125 KHz switching frequency. Embodiments herein have presented a different approach to the design of a CCM flyback converter.

The following steps and equations are adhered to while designing the 135V@0.75 Amps flyback converter in CCM.

Since the current in the primary switch SW 103 under CCM operation does not start from zero value at turn on, assuming a current ripple of 20% of the peak value, the average input current is computed as

I _(pavg)=0.9*I _(pp) *D   (15)

At minimum input voltage of 200V DC, assuming an efficiency of 90% I_(pavg) is

112/200=0.56 A   (16)

Limiting the duty cycle D to a maximum value of 0.5 at minimum input voltage, I_(pp) is computed as 1.24 Amps. Therefore the current ripple is 0.248 Amps.

L _(p)=200*4*10⁻⁶/0.248=3.2 mH   (17)

The turns ratio of the transformer can be calculated indirectly by first arriving at the secondary inductance L_(s).

Secondary average current I_(savg) is same as the load current I₁ which is 0.75 Amps.

I _(savg)=0.9*I _(sp)*(1−D)=0.45*I _(sp) at V _(inmin).   (18)

I _(sp)=0.75/0.45=1.66 A   (19)

N _(p) /N _(s) =I _(sp) /I _(pp)=1.66/1.24=1.34, say 1.5   (20)

Secondary inductance L _(s) =L _(p)/1.5*1.5=1.4 mH   (21)

The transformer primary turns are calculated as per the standard transformer equation based on the selected core cross sectional area and the frequency of operation. After finding the primary turns, the core is appropriately gapped to arrive at the needed primary inductance and so also the secondary inductance.

In the above condition, since the current ripple in the secondary is

0.2*1.66=0.332 A   (22)

The load current at which the CCM turns in to BCM is computed as

I _(1 ccm)=0.5*(1−D)*0.332=0.083 A at V _(inmin)   (23)

A flyback converter so designed for CCM was fabricated and evaluated for its performance.

The transformer core, power devices and all other components, excepting the control scheme and controller IC were similar for both boundary control and CCM for 100 W output power. The test data clearly depict the improvement in the efficiency, despite the fact that BCM draws higher peak currents. Input power saving was 3.0 watts at 400V input. At lower inputs, the power saving was minimal. However, the power loss in the main MOSFET switch was significantly minimized with the proposed technique, where by the power supply was run at 100 W output without providing any heat sink for the MOSFET SW 103. The rise in case temperature was only 25 degree centigrade above ambient suggesting a power dissipation of about 600 mill watts in the SW 103 at the full load of 100 W. However, in the CCM mode, the power loss in the SW 103 was much larger and it could not be run without heat sink for the SW 103 and when tried, the thermal run away started and increase in the input current was observed at which point the test was terminated. In BCM, the frequency varied from 125 kHz to 56 kHz with input while maintaining the complete soft switching of the power devices. Due to soft switching and absence of reverse recovery of the output diode lower levels of EMI are expected in BCM operation. At higher output voltages, absence of reverse recovery in diode is a great advantage because high voltage fast recovery diodes tend to exhibit higher recovery times, which in turn can adversely affect the efficiency figures. Alternate diodes like Silicon Carbide can be prohibitively expensive for low to medium power applications.

Embodiments herein do not warrant extra power/active devices, while having the advantages such as single stage conversion, input output isolation, and no dissipative snubber.

Embodiments disclosed herein can be used for a variety of applications including power factor correction. Embodiments disclosed herein can be used in applications such as LED street lights, automotive, earth moving equipment lamps, LED drivers, bias converters, heating elements, fan supplies, FHP motors, and so on.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. 

We claim:
 1. A flyback converter (100) comprising a switch (SW) (103) connected to primary windings of a transformer (T) (104); a capacitor (C_(r)) (101) connected across the switch (SW) (103); and a diode (D_(s)) (105) connected to the secondary windings of the transformer (T) (104); wherein a control input to the flyback converter (100) comprises a fixed dead time in a Boundary conduction Mode (BCM), wherein the fixed dead time is introduced before turning ON the SW (103).
 2. The flyback converter, as claimed in claim 1, wherein the SW (103) is at least one of a MOSFET switch, a bipolar transistor, and an insulated-gate bipolar transistor (IGBT).
 3. The flyback converter, as claimed in claim 1, wherein BCM is implemented with a variable switching frequency, wherein the switching frequency varies with line and load.
 4. The flyback converter, as claimed in claim 1, wherein C_(r) (101) absorbs the energy in the leakage inductance of the transformer (104).
 5. The flyback converter, as claimed in claim 1, wherein C_(r) (101) resonates with primary inductance of the transformer (104).
 6. The flyback converter, as claimed in claim 1, wherein turns ratio of transformer (104) is selected such that the reflected voltage at primary windings of the transformer (104) is at least equal to highest input voltage, while the diode D_(S) (105) is conducting.
 7. The flyback converter, as claimed in claim 1, wherein the fixed dead time is equal to the half of resonant time period of the primary inductance of the transformer (104) and C_(r) (101).
 8. The flyback converter, as claimed in claim 1, wherein voltage at C_(r) (101) is at least equal to twice the input voltage, when D_(S) (105) is conducting. 